Acrylic resins are being used in glazing applications such as windowpanes because of their high transparency. However, acrylic resins inherently have low rigidity, hardness and thermal resistance, and are hence less than satisfactory.
In an attempt to overcome this disadvantage, many investigations have heretofore been made on the formation of composite materials consisting of acrylic resins and inorganic substances. For example, there have been proposed a number of methods in which a dispersion of a silica compound (formed by polycondensation of an alkoxysilane) or colloidal silica in an acrylic resin solution is used as a coating film for hardening the surfaces of plastic substrates (see, for example, Japanese Patent Laid-Open Nos. 11952/'78 and 11989/'78).
However, when such a composite material is coated on plastic substrates, a coating film having high hardness and high wear resistance is obtained, but no substantial improvement in rigidity can be expected. Moreover, good transparency is obtained at coating film thicknesses of the order of several tens of microns, but a marked reduction in transparency results at greater coating film thicknesses.
On the other hand, it is described in J. Mater. Res., Vol. 4, p. 1018 (1989) that a silica gel-polymethyl methacrylate composite material is obtained by impregnating porous silica gel having a controlled pore diameter with methyl methacrylate and then polymerizing the latter. However, this method has the disadvantage that it involves troublesome steps and is not suitable for industrial purposes and that it is difficult to subject the resulting composite material to postworking.